A Novel Scheme of Digital Instantaneous Automatic Gain Control (DIAGC) for Pulse Radars Sumanta Pal, Nirmala Shanmugam, Mohit Kumar, P Radhakrishna Electronics and Radar Development Establishment Defense Research and Development Organization Bangalore – 560093, India Abstract - Several schemes for gain control are used for the saturating clutter zones and doesn’t affect the detection at preventing saturation of receiver, and overloading of data other zones. processor, tracker or display in pulse radars. The use of digital The remainder of the paper is organized as follows. processing techniques open the door to a variety of digital Section 2 provides information about different gain control automatic gain control schemes for analyzing digitized return techniques including their limitations. Section 3 presents the signals and controlling receiver gain only at saturating clutter zones without affecting the detection at other zones. In this DIAGC scheme including hardware implementation details paper, we present a novel scheme of Digital Instantaneous and data processing operation. Section 4 shows subsystem Automatic Gain Control (DIAGC) which is based on storing and system measurements and plots. Section 5 shows the digitally the dwell based clutter returns and deriving the gain effect of DIAGC on pulse compression. Finally, section 6 control. The returns corresponding to first two PRTs in a dwell provides several concluding comments followed by future are used to analyze the presence of saturating clutter zones and plans in section 7. the depth of saturation. Third PRT onwards proper gain control is applied at IF stage to prevent saturation of the following II. GAIN CONTROL TECHNIQUES stages. FPGA based scheme is used for digital data processing, storing, threshold calculation and gain control generation. The effect of DIAGC on pulse compression is also addressed in this A radar must be designed to produce sufficient signal from paper. the smallest target at its maximum range as well as to prevent saturation of receiver from large or close targets and clutters I. INTRODUCTION which in turn floods the display screen. When receiver saturates, it starts generating modulation products which Clutter cancellation is a major problem in pulse radars with spread into otherwise clutter free spectral region. This can be low-level coverage, especially when radar is required to avoided by distributing the gain throughout the receiver chain detect low RCS target in a dense clutter environment. The with successive steps of gain control. From the point where finite dynamic range of any subsystem in the radar implies gain control is applied, it helps prevent saturation in all that the system will be saturated by large clutter returns i.e. following stages. the strength of clutter returns exceed the available dynamic range. In this condition, radar receivers with fixed gain exhibit objectionable characteristics. For MTI radars (using classical MTI canceller or Doppler filter bank), it has a severe effect on clutter cancellation in the signal processor. The un-cancelled clutter residues can result at the output of Fig. 1. Distribution of gain control the delay line canceller producing large number of detections at the signal processor which eventually increases the load on There are several techniques available to control the data processor and floods the display screen. receiver gain, which are discussed along with their The effect can be prevented by some well known limitations in the following section. techniques such as Sensitivity Time Control (STC), Automatic Gain Control (AGC), Instantaneous Automatic A. Sensitivity Time Control (STC) Gain Control (IAGC), raising the antenna beam etc. But all of them have their inherent limitations. Conventional clutter STC is a technique by which the sensitivity of radar map formed apriory also may not hold good at the time of receiver is varied with time (range) to achieve more dynamic application (e.g. weather clutter). range. Maximum sensitivity is provided at long ranges, where In this paper, we present a novel scheme of DIAGC which it needs to detect the weak echoes of distant targets while the can take care of surface clutter as well as volume clutter at strong return from short ranges is prevented from saturating required range, azimuth and elevation (r, θ, φ) within the the system. Generally, this is achieved by applying coverage. The scheme prevents saturation of receiver only at attenuation in the RF front end. When the attenuation is applied at a rate 12 dB/octave in range, the amplified radar echo strength becomes independent of range. Other types of from the receiver and display, leaving any targets that may be STC laws are also used in radar such as 9 dB/octave (or R3 in the noise area. 2 law) to handle surface clutter, 6 dB/octave (or R law) to handle volume clutter etc. III. DIAGC SCHEME The limitation of STC is that the medium PRF and high PRF pulse radars cannot employ STC to reduce clutter effects The DIAGC operates on dwell basis. A number of pulses at short ranges unlike low PRF radars. The incompatibility of are transmitted in a dwell towards a particular azimuth and the STC requirements at elevation extremes severely limits elevation angle. The returns from each transmitted pulse are the usefulness of STC especially in multi-beam radars. integrated over the dwell time and detection reports are Moreover, STC cannot be applied at selective zones (r, θ, φ) generated in the signal processor. With the DIAGC, first two where saturating clutter is present. PRTs in a dwell are used to get an idea about the presence of saturating clutter returns and the depth of saturation. Third PRT onwards sufficient attenuation is applied (receiver gain is decreased) to prevent saturation in the following stages. On the other way, the DIAGC allows full amplification of weak signals and decreases the amplification of strong signals. As the attenuation controls are generated after digitizing the detected IF, hence the scheme can be called as DIAGC. Fig. 2. Example of STC law. B. Automatic Gain Control (AGC) AGC is a technique which controls receiver gain automatically as and when the radar return signal changes in amplitude. The simplest type of AGC adjusts the receiver Fig. 4. DIAGC scheme gain according to the average level of the received signal. Many surveillance radars employ an analog feedback loop A. RF Hardware Implementation configuration to achieve the required AGC action. AGC is generally applied at IF stages in a radar receiver. Throughout the paper, we have considered a pulse radar with double down-conversion receiver. The receiver chain consists of RF front end, down converter and IF receiver. RF front end contains all waveguide components followed by low noise amplifiers (LNA). STC is applied at the RF front end before the LNA. The down converter section consists of mixers followed by IF amplifier prior to pulse compressors (Pre-IF). The IF receiver section consists of pulse compressors, post IF amplifiers and I/Q demodulators. The Fig. 3. Conventional AGC system. dynamic range available for gain control is maximum near the RF front end and decreasing towards I/Q detector. When The response time of conventional AGC systems limit its only one IF stage is controlled, the range of DIAGC is usefulness. If the received power varies rapidly, the AGC limited to approximately 20 dB. When more than one IF loop becomes unstable which can degrade tracking accuracy. stage is controlled, DIAGC range can be increased approximately by 40 dB. C. Instantaneous AGC (IAGC) For our study and implementation, we have selected the Pre-IF stage for DIAGC as the requirement for gain control When AGC is performed on a pulse-to-pulse basis, it is was approximately 16 dB. The Pre-IF amplifier in the called Instantaneous AGC (IAGC). AGC varies gain based receiver is a gain controlled amplifier. The gain control is on the return from a broad area while IAGC allows mapping achieved by applying attenuation inside the pre-IF amplifier of the high noise area. IAGC subtracts the power of the first module. 6 bit control is available in the amplifier which pulse from the second; if the noise is uniform it will be erased corresponds to 31.5 dB of attenuation. These 6 bit controls are generated by DIAGC card, which is the digital hardware The 14 bit data received by the FPGA is passed through an module for the DIAGC. The amplifier gain in the coupled 8 range-bin wide moving average filter for the first two PRTs path is adjusted in such a way that detector dynamic range in a dwell. During the first PRT, moving average data is matches with the gain control range. stored inside the FPGA on range bin basis. During the next PRT, the stored data is read back and averaged with the current moving averaged data and again stored in the FPGA. Dual port block RAM cores are used as the memory element in the FPGA. Port A is used to write into the memory whereas port B is used to read from the memory. The control signals required for read and write operations are generated by the FPGA. Fig. 7. Dual port block RAM core When the third PRT comes, two PRT moving averaged Fig. 5. RF hardware for DIAGC data is read back from the memory and passed through threshold logic to generate proper attenuation control on B. Digital Hardware Implementation range-bin basis. This process repeats for the rest of the PRTs in a dwell. Response time between the third PRT memory The digital hardware for the DIAGC is a FPGA based read operation and the change in Pre-IF amplifier gain is card. It mainly contains a Xilinx Virtex FPGA (XCV200 calibrated and correction factor is applied.